IIRC at Stratford, the westbound Central line has platforms both sides and still opens doors both sides, one side for boarding and the other for exiting.
Free boarding on both sides at Stratford.
The LU examples are mostly (all?) at terminii (eg Morden) or turn-back locations (eg Golders Green), so there is going to be a pause anyway.
Yes and no. With the exception of Stratford they are all at termini or turn-backs, but many of those turn-backs can be used for through running. Even then, not all locations with platforms on both sides will open on both sides.
Canary Wharf on the DLR also has that set-up, which puzzled me the first time I used the station. My guess was that it's to facilitate cross-platform interchange between trains from Lewisham to Bank and trains to Stratford, though I'm not sure if that's correct.
The key difference with the DLR is, of course, that it uses bottom contact 3rd rail, so there is less of a danger of coming into contact with it should someone go down the gap (which is also pretty much non-existent on the DLR).
I don't know what happens at Barking Platform 2 now - with S Stock and with Platform 1 (which formerly served the GOBLIN) now being mostly disused.
With D Stock, however, at some point over the year, a change in procedures occured. Previously, the doors on the left hand side travelling east (on the unnumbered, narrow, platform joined to Platform 1)were closed before the doors on Platform 2 itself. Later on (certainly by the time that the GOBLIN was incorporated into the Overground) the opening and closure of both doors seemed usually to occur simulatneously on both sides, despite the unnumbered side being increasingly frequent used.
Barking has evolved in recent years, due to a combination of step-free access and a difference in how S stock and C/D stock work in terms of closing up. Whilst both stocks interacted there was a mix in process.
For all trains running east (District line), doors will open on both sides to provide easiest interchange, and access to the lift on platform 1. For trains terminating at Barking at reversing via the sidings (all H&C and some District), doors will open on platform 2 only (in theory), as there are no longer porter buttons on the exterior of cars to close up. This expedites the tip-out process as the car door passenger buttons are now used, with a slight delay to close up each car.
There isn't a standard around whether in-service trains close both sides simultaneously or staggered - this would be driver preference, depending on their view of the PTI.
At turnback locations like Arnos Grove, the double-sided centre road is for terminating trains and allows passengers wanting to continue to the terminus to get the next train cross-platform and gives London-bound passengers all of their services from one platform.
At places like Arnos doors are generally opened initially only on the appropriate side for customers to continue their journey cross-platform. After an appropriate amount of time, these are closed and the other side opened for new passengers to board for the other direction (generally into town).
Central line @ Stratford & District line @ Barking
Full list of LU 'double-sided' platforms and their usual usage:
- Arnos Grove centre road (2&3) - one side at a time, for direction of travel
- Barking (unnumbered/2) - eastbound only, as described above
- Cockfosters centre road - all trains, platform 3 only
- Golders Green centre road (3&4) - one side at a time, for direction of travel. Platform 1 is not in public use
- Loughton centre road (2/3) - one side at a time, for direction of travel.
- Morden centre road (3&4) - all trains. Platform 1 is not in public use
- Stratford (3/3a) - westbound only, both sides all trains
- Uxbridge centre road (2) - all trains, platform 3 only
- White City centre road (2&3) - reversing trains only, one side at a time